Sleep (non-human)

  Sleep in non-humans refers to how the behavioral and physiological state sleep, mainly characterized by reversible unconsciousness, non-responsiveness to external stimuli and motor passivity, appears in different categories of animals.

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About sleep

Sleep designates a behavioral and physiological state in organisms, characterized by:

• a reversible unconsciousness,
• non-responsiveness to external stimuli, and
• passivity of skeletal muscles (possibly with some exceptions; see below regarding the sleep of birds and of aquatic mammals),

where:

• deprivation of the state leads to a compensatory increase later (recovery sleep), and
• the individual has a body posture typical for sleep for its species^[1][2].

This behavioral definition of sleep differs from a neurophysiological definition of sleep, where sleep is defined by brain activity, eye movements and muscle tone^[3][4]. Animals kept from sleeping die within a couple of weeks, but the exact function of sleep is still unknown.

Sleep in different species

Organisms need some kind of nervous system to sleep. In very simple animals, behavioral definitions of sleep are the only ones possible, and it is not certain that the simplest animals even when active can show the behaviors that are normally considered to be coupled with being awake in higher animals.^[5]

Sleep in invertebrates

The electrophysiological study of sleep in small invertebrates is complicated. However, even such simple animals as e.g. fruit flies sleep, and if this sleep is disturbed systematically it leads to cognitive disabilities.^[6] There are several methods of measuring cognitive functions in fruit flies. A common method is to let the flies choose whether they want to fly through a tunnel that leads to a light source, or through a dark tunnel. Normally, flies are attracted to light. But if sugar is placed in the end of the dark tunnel, and something the flies dislike is placed in the end of the light tunnel, the flies will eventually learn to fly towards darkness rather than light. Flies deprived of sleep require longer time to learn this and also forget it more quickly. If an arthropod is experimentally kept awake longer than it is used to, then its coming rest period will be prolonged. In cockroaches that rest period is characterized by its antennae being folded down and a decreased sensitivity to external stimuli.^[7] Sleep has been described in crayfish, too, characterized both by passivity and increased thresholds for sensory stimuli as well as changes in the EEG pattern, markedly differing from the patterns found in crayfish when they are awake.^[8]

Sleep in lower vertebrates

Sleep in fish is not extensively studied. It has not been possible to prove that all species of fish sleep.^[9] There is doubt particularly about certain blind species that live in caves, for instance troglobites.^[10] Other fish sleep, however. Carp, for instance, like many higher vertebrates, show an increased need for sleep and fall asleep more quickly after being kept awake longer than usual.^[11]

Reptiles have been subjected to electrophysiological studies of sleep. That is to say that electrical activity in the brain has been registered when the animals have been asleep. However, the EEG pattern in reptillian sleep differs quite a bit from what is seen in mammals and other higher animals.^[5] In reptiles, an increased amount of sleep following sleep deprivation has been seen and stronger stimuli are needed to awaken them when they have been deprived of sleep, as compared to when they have slept normally. This suggests that the sleep after sleep deprivation is compensatorily deeper.^[2]

Sleep in birds

There are big similarities between sleep in birds and sleep in mammals^[12], which is one of the reasons for the idea that sleep in higher animals with its division into REM and non-REM sleep has evolved together with warm-bloodedness.^[13]

Birds have both REM and non-REM sleep, and the EEG patterns of both have similarities to those of mammals. Different birds sleep different amounts, but the associations seen in mammals between sleep and variables such as body mass, brain mass, relative brain mass, basal metabolism and other factors (see below) are not found in birds. The only clear explanatory factor for the variations in sleep amounts for birds of different species is that birds who sleep in environments where they are exposed to predators have less deep sleep than birds sleeping in more protected environments.^[14]

A peculiarity that birds share with aquatic mammals, and possibly also with certain species of lizards (opinions differ about that last point), is the ability for unihemispheric sleep. That is the ability to sleep with one cerebral hemisphere at a time, while the other hemisphere is awake^[15]. When only one hemisphere is sleeping, only the contralateral eye will be shut; that is, when the right hemisphere is asleep the left eye will be shut, and vice versa.^[16] The distribution of sleep between the two hemispheres and the amount of unihemispheric sleep are determined both by which part of the brain has been the most active during the previous period of wake^[17] – that part will sleep the deepest – and it is also determined by the risk of attacks from predators. In ducks, the ducks near the perimeter of the flock are likely to be the ones that first will detect predator attacks. These ducks have significantly more unihemispheric sleep than those who sleep in the middle of the flock, and they react to threatening stimuli seen by the open eye.^[18]

Opinions partly differ about sleep in migratory birds. The controversy is mainly about whether they can sleep while flying or not. Theoretically, certain types of sleep could be possible while flying, but technical difficulties preclude the registration of brain activity in birds while they are flying.

Sleep in mammals

  As for birds, the main rule for mammals (with certain exceptions, see below) is that they have two essentially different stages of sleep – REM and non-REM sleep.

Sleep in monotremes

Since monotremes, egg-laying mammals, are considered to represent one of the evolutionarily oldest groups of mammals, they have been subject to special interest in the study of mammalian sleep. As early studies of these animals could not find clear evidence for REM sleep, it was initially assumed that such sleep did not exist in monotremes but developed after the monotremes left the rest of the mammals and became a separate, distinct group. However, EEG registrations of the brain stem in monotremes show a firing pattern that is quite similar to the patterns seen in REM sleep in higher mammals.^[19][20] In fact, the largest amount of REM sleep known in any animal is found in the platypus.^[21]

Sleep in aquatic mammals

Among others, seals and whales belong to the aquatic mammals. Seals are grouped in earless seals and eared seals, which have solved the problem with sleeping in water differently. Eared seals, like whales, show unihemispheric sleep. The sleeping half of the brain does not awaken when they surface to breathe. When one half of a seal's brain shows slow wave sleep, the flippers and whiskers on its opposite side are immobile. While in the water, these seals have almost no REM sleep and may go a week or two without it. As soon as they move onto land they switch to bilateral REM and NREM sleep comparable to land mammals, surprising researchers with their lack of "recovery sleep" after missing so much REM.

Earless seals sleep bihemispherically like most mammals, under water, hanging at the water surface or on land. They hold their breath while sleeping under water, and wake up regularly to surface and breathe. They can also hang with their nostrils above water and in that position have REM sleep, but they do not have REM sleep underwater.

The only whale in which REM sleep has been observed is the pilot whale.^[22] Other whales do not seem to have REM sleep, nor do they seem to have any problems because of this. One reason REM sleep might be difficult in marine settings is the fact that REM sleep causes muscular atony; that is to say, a functional paralysis of skeletal muscles that can be difficult to combine with the need to breathe regularly.^[23][24]

Particular aspects of sleep in certain species

Different animals sleep different amounts. Some animals, like bats, sleep 18-20 hours per day, while others, like giraffes, only sleep 3-4 hours per day. This cannot be explained only by genetics, as there can exist big differences even between closely related animals.

An animal's feeding habits are associated with its sleep length. The daily need for sleep is highest in carnivores, lower in omnivores and lowest in herbivores. Humans do not sleep unusually much or unusually little compared to other animals, but we sleep less than many other omnivores.^[24] Many herbivores, like Ruminantia (such as cattle), spend much of their wake time in a state of drowsiness, which perhaps could partly explain their relatively low need for sleep. In herbivores, an inverse correlation is apparent between body mass and sleep length; big animals sleep more than smaller ones. This correlation is thought to explain about 25% of the difference in sleep amount between different animals.^[24] Also, the length of a particular sleep cycle is associated with the size of the animal; on average, bigger animals will have sleep cycles of longer durations than smaller animals. Sleep amount is also coupled to factors like basal metabolism, brain mass and relative brain mass.

Unihemispheric sleep

Unihemispheric sleep refers to sleeping with only a single cerebral hemisphere. The phenomenon has been observed in birds and aquatic mammals, as well as in several reptilian species (the latter being disputed: many reptiles behave in a way which could be construed as unihemispheric sleeping, but EEG studies have given contradictory results). Reasons for the development of unihemispheric sleep are likely that it enables the sleeping animal to receive stimuli, threats, for instance, from its environment, and that it enables the animal to fly or periodically surface to breathe when immersed in water. Only non-REM sleep exists unihemispherically, and there seems to exist a continuum in unihemispheric sleep regarding the differences in the hemispheres: in animals exhibiting unihemispheric sleep, conditions range from one hemisphere being in deep sleep with the other hemisphere being awake to one hemisphere sleeping lightly with the other hemisphere being awake. If one hemisphere is selectively deprived of sleep in an animal exhibiting unihemispheric sleep (one hemisphere is allowed to sleep freely but the other is awoken whenever it falls asleep), the amount of deep sleep will selectively increase in the hemisphere that was deprived of sleep when both hemispheres are allowed to sleep freely.

The neurobiological background for unihemispheric sleep is still unclear. In experiments on cats, where the connection between the left and the right hemisphere of the brain stem is severed, the hemispheres show a desynchronized EEG where the two hemispheres can sleep independently of each other.^[25] In these cats, the state where one hemisphere slept non-REM and the other was awake, as well as one hemisphere sleeping non-REM with the other state sleeping REM were observed. Interestingly, the cats were never seen to sleep REM sleep with one hemisphere while the other hemisphere was awake. This is in accordance with the fact that REM sleep, as far as is currently known, does not occur unihemispherically.

Sleep in hibernating animals

Animals that hibernate are in a state of torpor, differing from sleep. Hibernation markedly reduces the need for sleep, but does not remove it. Hibernating animals end their hibernation a couple of times during the winter so that they can sleep.^[26]

References

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